Venting pipe size transitions

ed1931June 25, 2009

When installing toilet wastepipe, I used 3" as required but due to space limitations, I am transitioning to a 2" vent pipe. I believe I read in one of the earlier threads that the transition must occur a minimum of 6" above the overflow plane of the fixture which in this case would be the top rim of the toilet. Am I interpreting this correctly?

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The vent pipe must rise vertically from the drain line, There may be mo horizontal run of the vent line until it reaches an elevation of 6" above the flood level rim of the highest fixture served by that vent.


If you are under the IRC (International Residential Code) all structures are required to have one "Main Vent" which must run undiminished in size from the "Main Drain" through the roof. After that is achieved any additional auxiliary vents may be reduced to 1/2 the diameter of the line they serve but not less than 1-1/4".

I.E. If you are under the IRC and you are installing an auxiliary vent on a 3" line, the vent line may be 1-1/2".

Under the UPC (Uniform Plumbing Code) to size a vent line you must first determine the total DFU (Drainage Fixture Units) load of all the fixtures served by that vent line, then consult UPC Table T7-5 to determine the size of line required for the vent.

Under the UPC ALL VENTS must terminate through the roof.

UPC Table T-7-5 Vent Sizing Table

Maximum number of DFU's / Minimum vent size
Max.1 dfu/1-1/4"
Max.8 dfu's/1-1/2"..(no water closets)
Max.24 dfu's/ 2"
Max. 48dfu's / 2-1/2"
Max. 84 dfu's / 3"
Max. 256 dfu's /4"

Under the UPC you are not required to have a Main vent, however the combined aggregate cross sectional area of all vents must be equal to or greater than the cross sectional area of the house main drain.

Now to answer you original question. You are venting a 3" line which serves a water closet so the minimum allowable vent line is 2". The 2" vent line may begin directly at the 3" drain line, and it must rise vertically. No horizontal run until it reaches an elevation at least 6" higher than the flood level rim of the water closet bowl.

    Bookmark   June 25, 2009 at 4:52PM
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Thanks Lazypup,

OK, your last paragraph answers my question. The transition from the 3" wastepipe to the 2" vertical vent line is just above the sanitary tee for the toilet, so I am OK with that. There is no vertical to horizontal transition until 6 ft. above the toilet bowl. All new vents will be tied in to the main vent in the attic or if necessary routed through the roof. So looks like I'm good to go with the venting arrangement design. The new toilet has a 2" vent as I mentioned, the 2" shower wastepipe has a 1 1/2 in. vent and the 1 1/2 in. vanity and bathtub shared wastepipe has a 1 1/2 in. vent line. The wastepipe tie in to the 4" main drain line in the crawl space and the venting connection in the attic will be done by a licensed plumber under local code and inspection. I don't want to mess with plumbing new PVC to existing cast iron (That could be catastrophic!). Otherwise does what I described sound OK?

    Bookmark   June 25, 2009 at 6:05PM
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It is difficult to be sure without seeing the actual layout, but from your description I suspect that you have much more venting than is actually required. On the other hand, no one will ever cite you for having too much venting.

Typically an entire bathroom group can be vented with one vent line going vertical from the riser behind the lavatory sink providing the fixture arms are within the code designated maximum lengths.

    Bookmark   June 26, 2009 at 5:48PM
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First, under the IRC Plumbing Code, there is absolutely NO 'size rule' that requires a 'main vent' to be run 'undiminished in size' from the 'main drain' to the roof.

If there is such an imaginary rule, then LazyPup needs to either CITE IT or RETRACT his obvious FALSE CLAIM.

Second, there is no such "height rule" as LazyPup falsely suggests is in the IRC Plumbing sections.

LazyPup needs to get a grip and explain why he just hasn't actually READ and actually doesn't UNDERSTAND the IRC Plumbing Code he just doesn't comprehend AT ALL!

And I will thank him in advance for correcting his errors.

    Bookmark   June 27, 2009 at 12:00AM
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Perhaps if you would spend half as much time actually reading the codes as you do pouring through internet forums trying to pass yourself off as a plumber you would already know the code references.

Believe me, after spending over 25yrs teaching code classes you can rest assured I never make a reference to code unless I am prepared to prove it.

"Main Vent required for building drain" IRC-3102.1

"No horizontal dry vents less than 6" above the fixture flood level rim, except kitchen island vents" IRC-3104.4

And for the record the UPC has the same limitation on horizontal vents. Ref: UPC-905.3

    Bookmark   June 27, 2009 at 7:21AM
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Thank you for your very informative post. I was only looking to see if I could transition my vertical to horizontal vent four feet above the washer (the washer is the only fixture on this vent).

Thank you again.

    Bookmark   February 17, 2011 at 9:37AM
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